A teenage boy has been missing for a month after going for a run and never coming back.

Joe Keller, who turned 19 the day after he disappeared, was last seen 10 days into a 15-day road trip from Tennessee to the West Coast with two of his best friends Collin Gwaltney and Christian Fetzner.

On July 23, Joe and Collin went for an afternoon run in Antonito, Colorado, but agreed to meet at a ranch because Collin is a faster runner.

However, when Collin reached the spot he sat there for hours, and Joe never made it.

A month later searches with helicopters, horses, dogs, drones and infrared equipment have found no trace of Joe, in what authorities have branded a 'baffling' missing persons case.

Missing: Joe Keller, who turned 19 the day after he disappeared, was last seen 10 days into a 15-day road trip from Tennessee to the West Coast with two of his best friends Collin Gwaltney and Christian Fetzner

His parents Neal and Zoe flew to Colorado on July 24, Joe's birthday, to search for him.

The family has now offered a $50,000 reward for information that could help them track him down.

'We'd just like to know where he is, what has happened, whether he is alive or dead,' Neal Keller, Joe's father, told Fox News.

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He remembered the last time he saw his son: 'I said, 'Joe, y'all have fun, be safe and I'll see you when you get home'. That hasn't happened yet.'

Two officers from Bradley County Sheriff's Office, in the Kellers' home town, have joined the search in Conejos County.

'We will continue to gather any information locally that might assist Colorado authorities in the search for Joe Keller,' Bradley County Sheriff Eric Watson said.

The teenager was wearing these sneakers (right) when he disappeared the day before his 19th birthday

A month later searches with helicopters, horses, dogs, drones and infrared equipment have found no trace of Joe (pictured, left, with his friends), in what authorities have branded a 'baffling' missing persons case

Joe (left) is described as 5'7', 150 pounds, with an athletic build, and a British-sounding accent

Posters have been issued across Colorado and in the family's home town in Cleveland, Tennessee, where Joe volunteers at his local Salvation Army and coaches a swim team.

He is due to begin his sophomore year at Cleveland State Community College, majoring in business, in September. He recently decided he wants to become a math teacher, his mother Zoe said.

According to a GoFundMe page set up by Collin's family, Joe is 5'7', 150 pounds, has an athletic build, and a British-sounding accent.

Anyone with information is asked to call the police on 423-650-4924 or Crimestoppers on 719-589-4111.

Desperate search: Posters with pictures of Joe have been issued across Colorado and in the family's home town in Cleveland, Tennessee, where he volunteers at his local Salvation Army and coaches a swim team

This is a support poster posted outside Cleveland State Community College, where he majors in business